SYNDEPOSITIONAL FORMATION OF CALCAREOUS NODULES ON CRETACEOUS MUDDY SEA FLOOR: POTENTIAL MUDDY NODULE PALEOTHERMOMETRY

Paleothermometry is one of the most important proxies for paleoceanographers. Benthic foraminifers have been used for reconstructing paleotemperature on the bottom of the sea. They are excellent materials for calcareous ooze, while mudstone sequences shows lots of difficulty to apply this technique for terrigenous sediments distributed around Pacific. Calcareous nodules are commonly observed in mudstone sequences, however, no study discussed potential paleothermometry based on calcareous nodules. We described occurrences at outcrops, general configurations including their cut sections, carbon content (CC), total organic carbon, and carbon, oxygen and sulfate sulfur isotopes of nodules collected from Cretaceous strata in Hokkaido, Japan.

Some observations demonstrate calcareous nodules can be formed just below the sea bottom (JBSB). Structure suggesting consolidation JBSB includes burrows that eject calcareous material from nodule. Nodules consolidated associated with anaerobic oxidation of methane (identified by carbon isotope values) with sulfate reduction appear to be JBSB origin, too. Such nodules show the exactly same oxygen isotope values with that of benthic foraminifers. A bivalve fossil found on one of the methane seep nodules preserved aragonite of the shell and yielded close oxygen isotope temperature with that of host nodule.

CC and oxygen isotope (d¹⁸O) values had positive relation suggesting CC was controlled by the depth of nodule production. Nodules with lower carbonate content (<50%) exclusively show low d¹⁸O values and inappropriate for the sea bottom paleothermometry.

The cross-plot of the data can emerge "upper limit line" of d¹⁸O values. The paleotemperature based on that value could provide reliable temperature for the sea bottom. On the other hand, nodules with similar condition from the Oyubari area appeared to be recrystalized and inappropriate for paleothermometry. It might be derived from the difference of burial depth between sediments of the Haboro and Oyubari areas. Even if it was originally consolidated JBSB, strong compaction during burial would have caused permeation of pore water into the nodule. Carbon dioxide or bicarbonate ions derived from decomposed organic matter would have caused recrystallization of calcite with d¹⁸O as low as -10 permil in the nodule.